The present invention relates generally to accelerating or decelerating particles, such as, for example, atoms.
It is a well-known result of non-relativistic quantum mechanics that two neutral atoms or molecules will attract each other because of their uncorrelated dipole-dipole interactions. In a related manner, a neutral atom will be attracted to a nearby perfectly conducting surface because of the interaction with its own image. The force between a neutral atom and a perfectly conducting parallel plane was first explained in the case of isomeric atoms by Lennard-Jones in non-relativistic perturbation theory and it is typically referred to as the van der Waals force.
When the distance between the atom and the surface becomes much larger than the typical wavelength of atomic transitions for that atom, Lennard-Jones"" non-relativistic treatment of the force fails (as a consequence of the fact that the speed of light is not infinite). In fact, relativistic quantum mechanics is required for analysis of such a case. The relativistic analysis was first preformed by Casimir and Polder. In this regime, which is referred to as xe2x80x9cretarded,xe2x80x9d the intensity of the force of attraction decreases more rapidly than in the van der Waals, or so-called xe2x80x9cun-retardedxe2x80x9d case.
Since all forces of this type depend on the optical (dielectric) properties of the materials involved, they are also referred to as xe2x80x9cdispersionxe2x80x9d forces. There is a connection between atom-surface forces and surface-surface forces. The former can be viewed as the limit of the latter when the second surface is an extremely rarified layer of neutral atoms. Consequently, surface-surface forces can be explained as simply a large-scale manifestation of the atom-surface force.
This understanding, which comprises the xe2x80x9csource theoryxe2x80x9d of dispersion forces, is of relatively recent vintage. The first complete theory of surface-surface forces was given by Casimir in 1948 with the introduction of xe2x80x9czero point energy,xe2x80x9d with no mention of the Lennard-Jones potential.
Zero point energy or xe2x80x9cvacuum energyxe2x80x9d or xe2x80x9cZPExe2x80x9d is energy that is associated with a non-thermal radiation that is believed to be present everywhere in the universexe2x80x94even in regions that are otherwise devoid of matter and thermal radiation. This non-thermal radiation is believed to result from random fluctuations occurring at the quantum level that result in a continual creation and destruction of virtual particles. This radiation is often referred to as a xe2x80x9czero point field,xe2x80x9d or by the acronym xe2x80x9cZPF,xe2x80x9d and the energy that is associated with the field is the aforementioned zero point energy. It is now understood that all dispersion forces, whether they be Casimir, Casimir-Polder, van der Waals, etc., are closely related to one another and can be explained by means of mutually exclusive, although equally acceptable, theories.
Until recently, there has been relatively little experimentation in the area of dispersion forces. In the case of surface-surface forces, this is due to the fact that the interacting boundaries must be highly polished and must be closer than about 1 micron for the forces to even approach measurability. The technology required for such experimentation was not available until recently.
Experimentation as to atom-surface forces, however, has not been quite as problematic. In particular, to measure atom-surface interactions, an atomic beam is directed near a conducting cylindrical surface and the deflection of the atoms from the surface due to van der Waals forces is measured. This xe2x80x9catomic deflectionxe2x80x9d approach, which is referred to as the Raskin-Kusch experiment, has been practiced for the last forty years to investigate van der Waals forces.
Investigators continue to use the techniques of the Raskin-Kusch experiment to probe van der Waals and other forces. Of late, experimentation is being performed with slow moving atoms that are obtainable using the relatively recent techniques of atomic trapping and cooling. Slower moving atoms are much more readily detected thereby facilitating more sensitive measurements.
Illustrative embodiments of the present invention provide methods and apparat uses by which a particle is accelerated or decelerated based on particle-surface interactions. In particular, in some embodiments, particles traveling away from a surface are made to travel at either a higher or lower speed than that with which they approached the surface. This change in velocity is effected by prompting the particles to undergo atomic transitions during their interaction with the surface.
In accordance with some embodiments of the present teachings, to accelerate a particle, such as an atom that is in its ground state, the particle is xe2x80x9cexcitedxe2x80x9d (i.e., transitions to a higher energy level) on its way to a close approach with a surface. In one embodiment, this excitation is accomplished using a laser. When the particle is at or near its closest approach to the surface, it is then advantageously prompted to return to its ground state.
During the approach to the surface in the excited state, the van der Waals force between the particle and the surface is quite high. After returning to the ground state, the van der Waals force between the particle and the surface is substantially reduced. This reduced attraction results in a significant increase in particle velocity.
To decelerate a particle, such as an atom in its ground state, the particle is excited, but that excitation occurs just after closest approach to the surface. Thus, the outbound particle, now in an excited state, experiences a much higher van der Waals force with the surface than when it was inbound toward the surface at the round state. This increased attraction significantly decreases particle velocity.
In some embodiments, an apparatus for carrying out the present methods comprises a particle source, a particle collimator, a particle exciter, a surface, and a particle de-exciter.
Underlying the present invention is the discovery that by prompting a particle to undergo an atomic transition during its interaction with the surface, its outgoing speed (i.e., speed as it moves away from a surface) can be changed relative to its incoming speed. The method is consistent with the conservation of energy principle.
The methods and apparatuses described herein are an improvement on the Raskin-Kusch experiment. The apparatus used in Raskin-Kusch experiments is similar to the present apparatus, but it does not include a particle exciter (or de-exciter). As a consequence, particles, such as atoms, are not prompted to undergo an atomic transition during their interaction with the surface.
Since the particles in Raskin-Kusch do not undergo an atomic transition during their interaction with the surface, they emerge from the interaction with the surface with a changed direction but with no change in speed. The constant speed is due to the fact th at the van der Waals force behave s as a conservative force. As such, when a particular atom is at a large distance from the surface, its speed is the same as it was on the incoming leg of the trajectory.